The objective of our laboratory has been to elucidate molecular mechanisms in pathogenesis of the human paramyxovirus parainfluenza virus type 3 (HPF3). In this application we propose to apply our expertise in the study of paramyxovirus entry and fusion to Hendra virus, a new paramyxovirus that is potentially fatal in humans and is viewed as a possible agent of bioterrorism. For HPF3, our central hypothesis is that the interaction of the receptor-binding protein, hemagglutinin-neuraminidase (HN), with cellular receptor(s) is critical for several essential components of the viral life cycle -- binding, entry, fusion and release -- and regulates pathogenicity. The proposal for Hendra virus evolves directly from our demonstration that HPF3 HN receptor binding is the critical first step towards HN's role in fusion promotion, and results in triggering of F protein to mediate fusion. For Hendra virus, expression of the receptor binding protein (G) is required in order for the F protein to mediate fusion. Our overall hypothesis is that Hendra G binding to receptor, like HPF3 HN binding to sialic acid, triggers F protein to mediate fusion. The study of this mechanism and the development of assays to analyze the triggering/activation of F protein in Hendra virus should lead to strategies for interfering with this key step in viral entry. Triggerinq Hendra virus F to mediate fusion: the role of G in activating F (1) Use of physical or chemical probes that alter G-mediated activation of F to study the process of G triggering F. We will either alter G-receptor binding by substituting other proteins in place of G, or alter G-promoted fusion using synthetic peptides, lipids and/or temperature to affect the function of G in activating F. In these ways we will characterize the process and requirements for G to trigger F-mediated fusion. (2) Identifying residues in G that participate in activating F: the use of mutations in G protein that affect G's promotion of F-mediated fusion to study the mechanism of F activation. We will use information derived from related paramyxoviruses, including HPF3, to identify residues in Hendra G protein that affect G's ability to trigger F to fuse. The proposal takes advantage of assays and strategies that we and others have developed for related paramyxoviruses, and applies these to understanding the activation of fusion by Hendra F. The results of the proposed studies will provide important new information about the fusion and entry mechanism of anemerging pathogen threat, and lead to development of strategies to block pathogenesis by this agent.